The simplest way (no macro needed):

- Click in the table and select Table > Convert > Table to Text. In the
dialog, select to separate text with tabs (this should be the default).

- Go through the resulting text, either manually or with a Replace
operation, to insert a tab between the two parts of each date.

- Select all the text that belongs in the table. Click Table > Convert >
Text to Table, and choose Tabs as the separator.